Pressure sensitive adhesives, also referred to as “PSAs”, are known in the art and are commercially available. Some of the more common types of PSAs are formulations based on acrylates, polyurethanes, natural rubbers, synthetic rubbers, and silicones. These PSAs are typically formulated for end use and find utility in a wide variety of applications including tapes, labels, bandages, transdermal drug delivery systems (e.g. patches), laminating adhesives, and transfer adhesives.
Acrylate-based PSAs, also referred to throughout as acrylate PSAs, are broadly used in these applications due to the fact that they are relatively low in cost when compared to other PSAs, solubilize many types of functional drugs for transdermal patches, adhere well to a variety of different surfaces, and can be formulated to build adhesion to a surface, if necessary. The disadvantages of acrylate-based PSAs include poor high temperature performance, poor low temperature performance, inability to adhere to surfaces with low surface energies, and the potential to build excessive adhesion to the skin in medical tape applications which can result in painful removal for the user. Examples of such acrylate-based PSAs are disclosed in U.S. Pat. No. RE 24,906.
Silicone-based PSAs, also referred to throughout as silicone PSAs, are typically produced by either blending or condensing together a silicone resin and a silicone polymer, such as polydimethylsiloxane (PDMS). Silicone materials by nature are very stable at high temperatures and the low glass transition temperature (Tg) of PDMS (less than −115° C.) ultimately provides a PSA that can find use in temperatures ranging from −100° C. to 265° C. Silicone-based PSAs also have excellent chemical inertness, electrical insulating properties, biocompatibility, and the ability to adhere to low surface energy substrates such as silicone release liners, polytetrafluoroethylene, and fluorohalocarbon materials. The primary disadvantage of silicone-based PSAs is their high cost compared to other technologies. Other limitations include lower tack and limited adhesion build (when necessary) in comparison to acrylate-based PSAs. Examples of such silicone-based PSAs are disclosed in U.S. Pat. Nos. 2,736,721; 2,814,601; 2,857,356; and 3,528,940.
There have been many attempts to combine acrylate PSAs and silicone PSAs to gain the advantages of both technologies. As a more specific example of one particular application, silicone pressure sensitive adhesives are frequently applied in the transdermal drug delivery systems. As is known, these systems typically include an active agent and the silicone pressure sensitive adhesive. The active agent, for example a pharmaceutical drug, is for controlled transdermal delivery or release to a substrate, such as the skin of a user of the system. The pressure sensitive adhesive functions to maintain contact between the system and the substrate for extended periods of time such that the active agent can be delivered to the substrate. Examples of such systems can be found in U.S. Pat. Nos. 3,731,683; 3,797,494; 4,031,894; and 4,336,243. Due to the particular silicone pressure sensitive adhesives used, the transdermal drug delivery systems of this prior art do not sufficiently optimize the solubility of the active agent in the pressure sensitive adhesives. As a result, the rate at which the active agent is released from the system for delivery to the substrate and also the total amount of the active agent that is ultimately released and delivered to the substrate are not optimized in this prior art.
In U.S. Pat. Nos. 5,474,783; 5,656,286; 6,024,976; 6,221,383; 6,235,306; 6,465,004; and 6,638,528, all to Noven Pharmaceuticals, Inc., the solubility of an active agent in a transdermal drug delivery system is optimized by simply blending acrylate pressure sensitive adhesives and silicone pressure sensitive adhesives together in varying ratios. However, because the two, separate PSAs are not actually chemically reacted together, domains of one PSA form within the continuous phase of the other PSA. In essence, gross phase separation occurs between the silicone-based PSA and the acrylate-based PSA both in liquid form and upon drying. As is known in the art, phase separation is generally caused by the incompatibility of two dissimilar materials, such as in the simple example of oil and water. In this particular case, the lower surface energy of the silicone PSA becomes incompatible with the higher surface energy of the acrylate PSA and phase separation occurs. Phase separation is also commonly referred to as instability. This instability limits the effective use time of the acrylate pressure sensitive adhesive/silicone pressure sensitive adhesive blend prior to and during application before phase separation occurs. Also, upon drying and as the blend ages over time, the size of the domains can potentially change as the two distinct PSAs try to reach an equilibrium state. This can lead to changes in properties such as tack, skin adhesion, and release from liner with time.
In another example, JP 62-295982, to Toyoda Gosei Co. LTD, describes a mounting system for an automotive application that consists of a molding and a PSA made by combining a silicone-based PSA, an acrylate-based PSA, and a polyurethane and/or polyisocyanate crosslinker together. The purpose of this mounting system is to provide a composition to mount a molding to an automobile main frame. For the silicone-based PSA and the acrylate-based PSA to be put together, a third polymeric species, specifically the polyurethane and/or polyisocyanate crosslinker, must be used to react the separate phases together. The disadvantages of this system include the requirement for the third polymeric species, a limited formulated pot life due to immediate reaction of the crosslinker, and unstable shelf-life stability of the coated product as the system will continue to crosslink with age, i.e., over time.
U.S. Pat. No. 4,791,163 to General Electric Company describes an emulsion that comprises (a) 100 parts by weight of water; (b) 10 to 400 parts by weight of PSA comprising: (i) from about 50 to 99% by weight of an organic PSA; (ii) from 1 to about 50% by weight of a silicone PSA; and (c) an effective amount of emulsifying agent to maintain the emulsion. The silicone-based PSA in solvent is first emulsified and then subsequently added to the organic PSA to provide the final composition. In this example, it is necessary to have careful control of the emulsifying agent and drying conditions to prevent premature phase separation of the emulsion prior to and during the drying step. Once the emulsion has been dried, there is no actual chemical reaction that occurs between the silicone PSA and the organic PSA.
Another example, EP 0 289 929 B1 also to the General Electric Company, describes the same emulsion as in the '163 patent with the addition of an effective amount of organic peroxide or alkoxy silane crosslinking agent to increase the shear strength of the emulsion through crosslinking within the silicone phase. Again, the emulsion requires the careful control of the emulsifying agent to prevent gross phase separation of the emulsion prior to and during the drying step.
In another example, JP 63-291971, to Nitto Electric Ind. Co. LTD, describes an adhesive that comprises a mixture of a silicone PSA, an acrylate PSA, and a silicone-acrylic graft copolymer. The silicone-acrylic graft copolymer is formed by the reaction of a silicone macromonomer with acrylic monomers during a polymerization reaction. The silicone-acrylic graft copolymer is then added to a blend of silicone PSA and acrylic PSA composition to act as a compatibilizer between the two different PSA phases. Because there is no actual chemical reaction between the PSAs, there still remains the potential for phase separation.
In WO 92/20751 to Minnesota Mining and Manufacturing Company (3M), a pressure sensitive adhesive composition preferably consists of acrylic monomer, a silicone pressure-sensitive adhesive, optional photoinitiator and optional crosslinker. Another series of 3M patents relating to vibration damping disclose this same composition (see WO 92/20752, and U.S. Pat. Nos. 5,464,6659 and 5,624,763). The goal of these compositions is to provide a solventless, radiation curable composition for use in PSA or vibration damping applications. The commercially available silicone PSA is first dried of all solvent. The solid silicone PSA mass is then dissolved in the desired monomer(s) followed by the addition of the photoinitiator and the crosslinker. The composition is then coated onto a substrate and cured into a final product by exposure to UV radiation. Although a crosslinker can be added, the composition is essentially an interpenetrating network where the acrylic monomer reacts while dispersed within the preformed silicone PSA network. The advantages of this composition are the ability to control the silicone PSA to acrylate ratio and also the ratio of acrylate monomer(s) within the acrylate itself depending on final use properties. As is clearly stated throughout these patents, the components of the composition are selected such that when the silicone PSA has been dispersed into the monomers to form a homogeneous mixture, the components will not exhibit phase separation when left at room temperature over a period of 12 hours. This still is a disadvantage in the fact that the materials will eventually phase separate with time. Another distinct disadvantage is that this system is typically cured in a substantially oxygen-free atmosphere or a nitrogen atmosphere. Therefore, handling becomes more complicated and pot-life of the formulated material becomes limited. Lastly, the use of photoinitiators in the UV-curing composition and their potential by-products almost certainly precludes its use in applications such as transdermal drug delivery systems which are loaded with active agents that could react or degrade in such environments. As alluded to above, there remains a need to improve upon the pressure sensitive adhesives of the prior art.